When you haven't still generated the file, `generated_user_id` is not filled, and
so, you can't control the language in which the payment order is going to be printed.
Adding a condition for using the user language in that case makes this more
comfortable.
* Removed deprecated YAML tests.
* Removed document dependency, as now there's no attachment dropdown.
* Switched deprecated `compute` method of res.currency.
* Restored missing field `reference_type` in invoices.
* Fixed call to `message_post`.
* Added missing description in model.
context of payment orders, she should ocus on the amount that
is due to be paid. In this method we are forcing to display both
the amount due in company and in the invoice currency.
We then hide the fields debit and credit, because they add no value.
